In recent years, hybrid electric vehicles have achieved commercial success in consumer automotive markets. This type of vehicle can improve overall energy efficiency by recapturing energy used by the vehicle's braking system, which is typically released to the environment as wasted energy for conventional (i.e., non-hybrid) vehicles. The captured energy is stored as electrical energy in batteries, which can be released to the drive system at appropriate times to reduce the load on the internal combustion engine of the vehicle. Certain types of hybrid electric vehicles can achieve economy improvements of 30% or more.
While hybrid electric vehicles have gained popularity for consumer vehicles (e.g., sedans, minivans, SUVs, etc.), such vehicles are limited in their ability to provide desirable fuel economies for larger and heavier vehicles, such as military vehicles and large package transport vehicles. The high life-cycle cost associated with required battery replacements (and the resulting carbon footprint associated with disposing or recycling used batteries) as well as the larger and heavier battery that is required for larger and heavier vehicles makes a hybrid electric system undesirable for such vehicles. In addition, the limits in power delivery of battery systems also render the hybrid electric technology undesirable for use with vehicles that are larger and heavier in relation to typical consumer vehicles.
Hybrid hydraulic vehicles have also been utilized for motorized vehicles, in which a compressed gas is used to pump hydraulic fluid into a hydraulic motor to provide assistance to the combustion engine of the vehicle. However, there are environmental concerns with such systems, such as hydraulic fluid leakage as well as economic costs associated with the additional weight and maintenance associated with such systems.